LPC1766 datasheet(59/66 Pages) NXP

59 / 66 page

LPC1769_68_67_66_65_64_4

Product data sheet

Rev. 04 — 1 February 2010

59 of 66

NXP Semiconductors

LPC1769/68/67/66/65/64

32-bit ARM Cortex-M3 microcontroller

14.2 XTAL1 input

The input voltage to the on-chip oscillators is limited to 1.8 V. If the oscillator is driven by a

clock in slave mode, it is recommended that the input be coupled through a capacitor with

Ci = 100 pF. To limit the input voltage to the specified range, choose an additional

capacitor to ground Cg which attenuates the input voltage by a factor Ci/(Ci + Cg). In slave

mode, a minimum of 200 mV (RMS) is needed. For more details see the LPC17xx User

manual.

14.3 XTAL and RTCX Printed Circuit Board (PCB) layout guidelines

The crystal should be connected on the PCB as close as possible to the oscillator input

and output pins of the chip. Take care that the load capacitors Cx1, Cx2, and Cx3 in case of

third overtone crystal usage have a common ground plane. The external components

must also be connected to the ground plain. Loops must be made as small as possible in

order to keep the noise coupled in via the PCB as small as possible. Also parasitics

should stay as small as possible. Values of Cx1 and Cx2 should be chosen smaller

accordingly to the increase in parasitics of the PCB layout.

Fig 27. LPC1769/68/66/65/64 USB device port configuration

LPC17xx

USB-B

connector

002aad943

USB_UP_LED

USB_CONNECT

VDD

D+

−

USB_D+

USB_D

−

VBUS

VSS

Fig 28. Slave mode operation of the on-chip oscillator

LPC1xxx

XTAL1

100 pF

002aae835

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